Messages in this thread Patch in this message | | | From | "Zi Yan" <> | Subject | Re: [RFC -mm] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration | Date | Mon, 06 Nov 2017 10:53:48 -0500 |
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On 4 Nov 2017, at 23:01, huang ying wrote:
> On Fri, Nov 3, 2017 at 11:00 PM, Zi Yan <zi.yan@cs.rutgers.edu> wrote: >> On 3 Nov 2017, at 3:52, Huang, Ying wrote: >> >>> From: Huang Ying <ying.huang@intel.com> >>> >>> If THP migration is enabled, the following situation is possible, >>> >>> - A THP is mapped at source address >>> - Migration is started to move the THP to another node >>> - Page fault occurs >>> - The PMD (migration entry) is copied to the destination address in >>> mremap >>> >> >> You mean the page fault path follows the source address and sees >> pmd_none() now >> because mremap() clears it and remaps the page with dest address. >> Otherwise, it seems not possible to get into handle_userfault(), >> since it is called in >> pmd_none() branch inside do_huge_pmd_anonymous_page(). >> >> >>> That is, it is possible for handle_userfault() encounter a PMD entry >>> which has been handled but !pmd_present(). In the current >>> implementation, we will wait for such PMD entries, which may cause >>> unnecessary waiting, and potential soft lockup. >> >> handle_userfault() should only see pmd_none() in the situation you >> describe, >> whereas !pmd_present() (migration entry case) should lead to >> pmd_migration_entry_wait(). > > Yes. This is my understanding of the source code too. And I > described it in the original patch description too. I just want to > make sure whether it is possible that !pmd_none() and !pmd_present() > for a PMD in userfaultfd_must_wait(). And, whether it is possible for > us to implement PMD mapping copying in UFFDIO_COPY in the future? >
Thanks for clarifying it. We both agree that !pmd_present(), which means PMD migration entry, does not get into userfaultfd_must_wait(), then there seems to be no issue with current code yet.
However, the if (!pmd_present(_pmd)) in userfaultfd_must_wait() does not match the exact condition. How about the patch below? It can catch pmd migration entries, which are only possible in x86_64 at the moment.
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 1c713fd5b3e6..dda25444a6ee 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -294,9 +294,11 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx, * pmd_trans_unstable) of the pmd. */ _pmd = READ_ONCE(*pmd); - if (!pmd_present(_pmd)) + if (pmd_none(_pmd)) goto out;
+ VM_BUG_ON(thp_migration_supported() && is_pmd_migration_entry(_pmd)); + ret = false; if (pmd_trans_huge(_pmd)) goto out;
— Best Regards, Yan Zi
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